Local coordination structure and electronic structure of the large electron mobility amorphous oxide semiconductor In-Ga-Zn-O: Experiment andab initiocalculations

Ionic amorphous oxide semiconductors (IAOSs) are new materials for flexible thin film transistors that exhibit field-effect mobilities of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ [K. Nomura et al., Nature 488, 432 (2004)]. The local coordination structure in an IAOS, In-Ga-Zn-O ($a$-IGZO), was examined using extended x-ray absorption fine structure analysis combined with ab initio calculations. The short-range ordering and coordination structures in $a$-IGZO are similar to those in the corresponding crystalline phase, $\mathrm{In}\mathrm{Ga}\mathrm{Zn}{\mathrm{O}}_{4}$, and edge-sharing structures consisting of In-O polyhedra remain in the amorphous structure. The ${\mathrm{In}}^{3+}$ $5s$ orbitals form an extended state with a band effective mass of $\ensuremath{\sim}0.2{m}_{e}$ at the conduction band bottom.